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Astron. Astrophys. 317, 637-645 (1997)

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HST polarization observations of the jet of M87 *

A. Capetti 1, F. D. Macchetto 1, 2, W. B. Sparks 1 and J.A. Biretta 1

1 Space Telescope Science Institute; 3700, San Martin Drive, Baltimore, MD 21218, USA
2 Associated with the Astrophysics Division, Space Science Department of ESA; Estec, NL-2200 AG Noordwijk, The Netherlands

Received 13 September 1995 / Accepted 6 March 1996


We present the results of pre-refurbishment HST polarization observations of the jet of M 87 taken with the Faint Object Camera in the ultraviolet and with the WF/PC-I in the visual. From these observations we produce polarization maps of unprecedentedly high resolution. The structure of the projected magnetic field is extremely complex and changes on the smallest observable scales. The degree of polarization increases from the center toward the edges of the jet where it reaches 60%. A comparison of these images with radio polarization maps shows no measurable depolarization or Faraday rotation.

The morphology of knot D is substantially different in the VLA observations by Owen et al. and these HST observations. The measured changes in knot D are inconsistent with proper motions and must be intrinsic to the jet structure. While the magnetic field configuration is remarkably similar at the locations of knots D2 and D3, the magnetic field appears to have changed from parallel to perpendicular at the location of knot D1. These results suggest that this component is associated with a shock front. Simultaneous polarization observations are required to determine if the changes in the magnetic structure are due to an intrinsic difference in the magnetic field configuration as seen by optical and radio emitting electrons or to a change of the jet structure, with the appearance of a new component.

Key words: galaxies: individual (M 87) – galaxies: jets – polarization

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

Send offprint requests to: A. Capetti

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998